The invention pertains to a method for packaging objects as defined in the preamble of claim 1 and to an apparatus for packaging objects as defined in the preamble of claim 6.
Methods and apparatuses of the generic kind are particularly used for packaging stacks of goods arranged on pallets, for securing the good to be packed against slipping on the pallet caused by the forces occurring during transport and to protect the stack of goods against humidity and other environmental influences. The foil hood for this purpose is stretched in transverse and longitudinal directions when being pulled over. I.e. it is elastically stretched in transverse direction prior to being pulled over and is elastically stretched in longitudinal direction during being pulled over. After having been pulled over, the foil hood on all sides resiliently bears on the stack of goods and the enclosed pallet. It reaches below the pallet. The foil hood compresses the stack of goods frequently consisting of individual bags or boxes and in its entity presses it against the pallet.
The plastic foils used for such packagins are elastically extensible to a high extent. However, they at the same time have high internal friction, i.e. the elastically extended foil after relief does not return into its relieved position in a xe2x80x9cbouncing movementxe2x80x9d completely. Return into the relieved position at least partly rather is effected in a continuous movement requiring some period of time.
From EP-A-0 633 186 a method and an apparatus for enveloping unit loads, in which a stretch foil is pulled over a stack of goods. For this purpose, at first a foil hose is welded at a desired length to form a foil hood and is cut above the welding seam. The thus created foil hood is reefed by means of reef rolls onto four gripping means tenting a rectangle and are transversely stretched into a shape being somewhat larger than the cross-section of the stack of goods to be packaged. Subsequently, the foil hood is pulled over the stack of goods by a downward movement of the gripping means and therein is stretched in longitudinal direction in that the foil is pulled away from the gripping means with the occurrence of substantial sliding friction. For increasing the friction of the foil during removal, i.e. for optimizing the vertical stretching, the foils is pressed against the gripping means by means of the reef rolls. The foils pulled down from the gripping means due to the high internal friction does not immediately solidly bear on the side wall of the charge. Rather is such solid contact fixing the foil on the charge in frictional connection, only effected after some time, i.e. with some distance from the gripping means which in the meantime moved further downwardly. In the lower region of the stack of goods, i.e. when reaching the lower edge of the pallet, therefore, the foil hood is not simply released, because in case of letting it go, the foil which did not yet contact the charge in frictional connection in the lower area would contract in horizontal as well as in vertical directions. The foil end would be pulled up above the bottom edge of the pallet into the lateral area of the pallet even before it could contract in horizontal direction so much that it reliably reaches below the pallet. In order to avoid this, a so-called under-stretch is actively created in the lower area of the stack of goods. This is effected in that the gripping means holding the foil end are moved in inward direction as compared to the contour for pulling over (i.e. the contour with the foil hood has during being pulled over), i.e. under the pallet in particular. By the inside movement of the gripping means the foil end still tented on the gripping means is discharged, i.e. the friction between the gripping means and the foil end still tented on them decreases. This increases the danger that the foil end slips from the gripping means already before completion of the understretch. In order to avoid this, the foil hood is pressed against the gripping means by means of the reef rolls and thus is safely held on the gripping means also in case of decreasing tension between the foil end and the latter ones.
It is true that in this method the vertical stretch is improved by pressing the foil hood by means of the reef rolls. However, an improvement only is possible to a certain degree, as an excessive pressure of the reef rolls acting on only a narrow area at the corners of the gripping means can cause damage of the foil during removal. Under certain circumstances holes or at least undesired plastically deformed areas can be created in the foil hood by excessive pressure of the reef rolls. In addition, pressing the foils to the grippers for safely holding the foil on the grippers during creation of the understretch includes disadvantages. The reef rolls cannot act on the outermost (seen in direction of pulling off of the foil) end of the frame sections which come to lie directly below the pallet bottom edge, at least not without additional constructional expense having to be accepted. For this reason, below the pallet bottom edge an unnecessarily long foil section which is not be required for the proper understretch per se has to be provided for only for making sure that the foil end reliably protrudes until in the area of the reef rolls.
From the DE90 01 319 U1 another apparatus for packaging goods is known in which movable clamping jaws are used for pressing the foil hood in a lower area against the stack of goods. Also this way of action prevents the foil from slipping upwardly, until the understretch is completed, however, it can easily cause undesired damaging of the foil hood and/or the good to be packed, respectively, and in addition also involves undesired additional constructional expense.
It is, therefore, the main of the present invention to avoid the above-cited disadvantages and to create a method and an apparatus for packaging objects with a stretch foil hood, in which the vertical stretch of the foil is improved with lowest constructional expense possible and an understretch of the foil hood can be reliably created with a very small excessive length of the foil with respect to the pallet bottom edge.
This object is in advantageous manner solved in accordance with the method in accordance with the present invention with the characterizing features of claim 1 and in accordance with the corresponding apparatus with the characterizing features of claim 6.
By the fact that the foil is guided over at least one recess of the slide surface, i.e. at least one section in the slide surface which is recessed as compared to the slide surfaces directly prior and subsequent thereto in direction of sliding, the resistance which the foil puts up against its being pulled off is optimized, because the foil contracts under its pretension in the region presently bridging the recess. Thereby, it ingresses into the recess. When leaving the recess, the foil again is deflected and again is stretched due to the positive lock with the area deflecting it. Herein, an intense frictional connection occurs. It is not absolutely necessary that the bow-shaped frame sections protrude over the adjacent contour of the frame support particularly.
In accordance with a particularly preferable embodiment of the method and the apparatus it is provided that the recess across which the foil is guided is built as gap between the bow-shaped frame section and the bow support. As the bow-shaped frame section and the bow support generally anyway represent two individual components, a gap can be realized in particularly simple manner by enlarging the separating gap therebetween.
In accordance with a particularly preferable further development of the method and the apparatus it is provided that the gap is or can be changed in its depth T and/or its length L prior to or during pulling off the foil. By the general adjustability, the gap can be adapted to the parameters important for each individual case, like foil material, foil thickness, pulling-over speed, pretension of foil, profile of the stack of goods to be packed etc. As the gap can be adjusted during the pulling-over movement, it becomes possible to account for e.g. friction conditions changing in the course of pulling-over, a charge contour changing in direction of pulling-over and also for other parameters changing in course of pulling-over. As the depth T of the gap (i.e. the distance from the deepest point of the gap to the highest point of the sliding surface directly following the foil in direction of pulling off) and the length L of the gap can be adjusted independently from one another, a particularly fine adaption to the respective conditions is possible.
In accordance with a particularly preferred embodiment of the method in accordance with the present invention the foil is guided such that the lower rim of the foil hood during the final phase of pulling over snaps into the gap and contracts such that the foil end at least temporarily is held during the movement of the gripping means to the inside with respect to the pulling-over contour for creating an understretch. Wrapping of the slide surface still being into contact with the foil is intensified by such guiding of the foil, friction is increased.
In correspondence with another embodiment of the method in accordance with the present invention the foil is guided over the gap such that it only partly is in contact with the surfaces bordering the gap. Thereby, folds in the foil can extended, before the foil runs over the bow-shaped frame sections.
In accordance with another embodiment of the apparatus in accordance with the present invention the at least one recess of the slide surface is located as close as possible to the end of the gripping means, at which the foil leaves the gripping means. Due thereto, an only very short foil section is sufficient for holding the foil on the gripping means, while those are moved in inward direction with respect to the pulling-over contour for creating an understretch. Foil is saved.
In accordance with another embodiment of the apparatus in accordance with the present invention the slide surface in leaving direction of the foil merges into the recess almost in a leap. Thereby, the snapping in of the foil end into the recess is favored which is in particular important for the holding of the foil during creation of the understretch, in that the foil end is relieved almost suddenly when reaching the recess.
Preferably, the apparatus in accordance with the present invention is realized such that the gap at each gripper means extends over the entire outer circumference thereof. Thus, during pulling off of the foil in the pulling-over phase an increased resistance occurs on the entire circumference of the bow-like frame section, this improving the vertical stretch. Moreover, this design can improve holding of the foil in the phase of creation of the understretch.